The present invention is related to solid-state imaging devices and cameras incorporating such devices. In particular, the invention pertains to such devices and cameras that can monitor the quantity of incident light.
FIG. 13(a) is a top view showing a typical composition of a picture element unit (pixel) in a conventional solid-state imaging device.
FIG. 13(b) is a cross section along the line 13bxe2x80x9413b shown in FIG. 13(a), and FIG. 13(c) is a cross section along the line 13cxe2x80x9413c shown in FIG. 13(a).
This solid-state imaging device is a frame-transfer type CCD picture element sensor (hereinafter referred to as simply xe2x80x9cCCDxe2x80x9d) having a lateral overflow drain construction (hereinafter referred to as xe2x80x9cLODxe2x80x9d).
As shown in FIG. 13(b), N-type semiconductor regions 2, 3 are formed on the surface of a P-type semiconductor substrate 1. As shown in FIG. 13(a), the N-type semiconductor regions 2 and 3 are alternately formed facing toward the longitudinal direction of the figure. The impurity concentration in the N-type semiconductor region 3 is higher than in the N-type semiconductor region 2. Further, as shown in FIG. 13(c), an N-type semiconductor region (LOD diffusion region) 4 and P-type semiconductor regions 5, 6 are formed on the surface of the P-type semiconductor substrate 1, and the N-type semiconductor region 4 is positioned between the P-type semiconductor regions 5 and 6.
An oxidation layer 7 is formed on the N-type semiconductor regions 2, 3, 4 and the P-type semiconductor regions 5, 6, and vertical-transfer gate electrodes 8a, 8b are formed on this oxidation layer 7. The vertical-transfer gate electrodes 8a, 8b comprise, for example, transparent polysilicon. As shown in FIG. 13(a), the gate electrodes 8a and 8b extend in the horizontal direction and are alternately wired facing toward the longitudinal direction.
A vertical-transfer portion 9 comprises the N-type semiconductor regions 2 and 3. This vertical-transfer portion 9 also functions as a light-receiving portion and is a region where the optical-signal charges Qs (electrons in this case) generated by light incident on the CCD are stored and transferred. Moreover, the P-type semiconductor region 5 functions as an anti-blooming barrier. Any excessive electrical charge Qex that cannot be stored in the-vertical-transfer portion 9 invades the region 5 and is discharged to the LOD diffusion region 4 (N-type semiconductor region). In addition, the P-type semiconductor region 6 functions as a pixel-separation region. The normal size of a pixel is approximately 5-20 xcexcm. Because a shading membrane that blocks light impinging on a pixel portion is not formed in this type of solid-state imaging device, light impinges on the entire pixel portion.
FIG. 14 is a compositional drawing showing a CCD wherein pixel units of the solid-state imaging device shown in FIG. 13(a) are arranged in a two-dimensional matrix. This CCD comprises a light-receiving portion 10 and a horizontal-transfer portion 11 comprising the vertical transfer portion 9 of FIG. 13(a).
FIG. 15 is a top view showing a typical end portion (and its periphery), of the vertical-transfer portion 9 shown in FIG. 13(a). Aluminum wiring 13 is connected through a contact region 9b to an N-type region 9a of the end portion of the vertical-transfer portion 9 positioned on a side opposite the horizontal-transfer portion 11. An electrical potential Vr is applied to the aluminum wiring 13. The potential Vr causes the bias of the vertical-transfer portion 9 to be opposite the bias of the P-type semiconductor substrate 1. Further, a gate electrode 8c is formed between the aluminum wiring 13 and the gate electrode 8b, and a constant electrical potential Vc is applied to the gate electrode 8c. Overflow wiring 14 is connected through a contact region 4a to the end portion of the LOD diffusion region 4, and a constant electrical potential Vf is applied from a terminal 18 through the overflow wiring 14 to the LOD diffusion region 4. The potential Vf has a bias opposite to the bias of the P-type semiconductor substrate 1.
In the CCD of FIG. 14, an optical-signal charge Qs is generated and stored in each pixel by means of light incident on the vertical-transfer portion 9. Next, the optical-signal charge Qs is transferred to the horizontal-transfer portion 11 and further transferred from the horizontal-transfer portion 11 to an output amplifier 12. The optical-signal charge Qs is then output to the exterior of the element as a signal Vout through the output amplifier 12. From among the optical-signal charges Qs, excessive electrical charge Qex that cannot be stored in the vertical-transfer portion 9 overflows into the LOD diffusion region 4. The excessive electrical charge Qex overflowing into the LOD diffusion region 4 is discharged through the overflow wiring 14 on which is applied a constant electrical potential Vf.
Next, a combined operation by the above-mentioned CCD and shutter to capture a still picture will be described using the drive-timing chart shown in FIG. 16.
First, a pre-exposure initialization is carried out in a state in which the shutter is closed during the period Tp1. Namely, in order to reset the electrical charge stored in the horizontal-transfer portion 11, a clock pulse (not shown in figure) is applied to the horizontal-transfer portion 11, and the electrical charges of the horizontal-transfer portion 11 successively transfer to the output amplifier 12. Thereafter, clock pulses xcfx86V1, xcfx86V2 are applied to the vertical-transfer gate electrodes 8a, 8b, and the electrical charges stored in the vertical-transfer portion 9 successively transfer to the horizontal-transfer portion 11. After an electrical charge transferred to the horizontal-transfer portion 11 in this manner is transferred within the horizontal-transfer portion 11, the electrical charge is output through the output amplifier 12 and is then reset. The vertical-transfer portion 9 and the horizontal-transfer portion 11 are reset by means of this sequence of transfer operations.
Thereafter, in the period Tp2, after the clock pulses xcfx86V1, xcfx86V2 applied to the vertical-transfer gate electrodes 8a, 8b are maintained at an xe2x80x9cLxe2x80x9d (low) level, the shutter opens and the CCD enters an exposure state. Because the impurity concentration in the N-type semiconductor region 3 is higher than the impurity concentration in the N-type semiconductor region 2, the optical-signal charge Qs is stored in the N-type semiconductor region 3 during the period Tp2.
Next, in the period Tp3, the shutter is closed, a clock pulse is applied to the vertical-transfer portion 9 and the horizontal-transfer portion 11; the optical-signal charge Qs is transferred, and a picture signal is output from the output amplifier 12 to the exterior of the element.
In other words, when xcfx86V1 becomes a xe2x80x9cHxe2x80x9d (high) level, the optical-signal charge Qs stored in the N-type semiconductor region 3 under the vertical-transfer gate electrodes 8a, 8b in the exposure state of the period Tp2 is collected in the N-type semiconductor region 3 under the vertical-transfer gate electrode 8a to which the clock pulse xcfx86V1 is applied. Thereafter, when xcfx86V2 becomes a xe2x80x9cHxe2x80x9d (high) level, the optical signal charge Qs of the N-type semiconductor region 3 is transferred to the N-type semiconductor region 3 under the vertical-transfer gate electrode 8b on which the clock pulse xcfx86V2 of an adjacent pixel is applied. The electrical signal charge in FIG. 13(a) is transferred in the upward direction. That is, when the clock pulse xcfx86V2 rises to a xe2x80x9cHxe2x80x9d (high) level, the optical-signal charge Qs is transferred from the vertical-transfer portion 9 to the horizontal-transfer portion 11. Before the clock pulse xcfx86V2 rises to a xe2x80x9cHxe2x80x9d (high) level, the optical-signal charge Qs of the horizontal-transfer portion 11 is transferred to the output amplifier 12 and a picture signal is output to the exterior of the element. In this manner, the optical-signal charges Qs of one picture portion are sequentially read out.
Because picture capturing is carried out using a pre-determined exposure time (i.e., period Tp2 of FIG. 16) in the above-mentioned conventional solid-state imaging device, if the quantity of incident light suddenly increases and changes from an estimated value during this exposure time, it will become impossible to read light information at an optimum exposure level. The following methods have been considered as methods to solve this problem.
As a first method, an exposure-control sensor is disposed at a position separate from an optical system (light-receiving portion) of a solid-state imaging device. The light intensity from an object to be photographed is monitored by the sensor and the exposure is adjusted accordingly. As a second method, an exposure-control sensor provided with a half mirror is disposed within an optical system of a solid-state imaging device. One portion of the light (i.e., light passing through a lens) incident on the optical system is removed by means of the half mirror. The removed light is monitored by the sensor and the exposure is adjusted accordingly. As a third method, an exposure-control sensor is disposed close to a light-receiving portion of a solid-state imaging device. Light incident on an optical system is reflected by the receiving portion. The reflected light is monitored by the sensor and the exposure is adjusted accordingly.
In the first method summarized above, however, because the intensity of light that is directly incident on the light-receiving portion of the solid-state imaging device is not monitored, there is a problem of poor accuracy of the exposure control. In the second method, because one portion of the light incident on the solid-state imaging device is removed by the half mirror, there is a problem of loss of one portion of the incident light, leading to unfavorable sensitivity. In the third method, because light incident on the solid-state imaging device does not undergo scattering, the intensity of the reflected light is greatly reduced. The resulting problem is that incident light is not monitored with good accuracy.
The present invention takes into consideration the above-mentioned facts concerning the prior art and has an object of providing a solid-state imaging device, and a camera comprising such a device, that can directly detect changes in light intensity incident to a light-receiving portion in real-time. Another object is to provide such a device and camera that can read light information at an optimum exposure level whenever estimates of the intensity of incident light are difficult to obtain and whenever the intensity of incident light suddenly changes from an estimated value.
In order to solve the above-mentioned problems, a solid-state imaging device according to a first embodiment of the present invention comprises a charge-transfer portion that generates and stores electrical charges in response to incident light, and transfers the signal charges. The device also comprises an output portion that outputs the signal charge transferred from the charge-transfer portion as an electrical signal. The device also comprises a semiconductor region that generates a signal charge in proportion to the quantity of incident light. The device also comprises a shading membrane having an aperture portion formed on the semiconductor region.
Further with respect to the first embodiment, a semiconductor region generates a signal charge in proportion to the quantity of incident light. Therefore, it is possible to generate signal charges, that are separate from the charge-transfer portion, in proportion to the quantity of incident light to the semiconductor region through an aperture portion. This causes a signal charge to be generated in the semiconductor region in real-time during an exposure. The signal charge is proportional to the quantity of light incident to the solid-state imaging device. Because of this, it is possible to detect changes in light intensity directly incident to the light-receiving portion in real-time and then read light information at an optimum exposure level whenever estimates of the intensity of incident light are difficult to obtain and whenever the intensity of incident light suddenly changes from an estimated value.
A solid-state imaging device according to a second embodiment of the present invention comprises a charge-transfer portion that generates and stores signal charges in response to incident light and then transfers the signal charge. The device also comprises an output portion that outputs the signal charge transferred from the charge-transfer portion as an electrical signal. The device also comprises a semiconductor region, formed adjacent to the charge-transfer portion, through which any excess signal charge generated by the charge-transfer portion flows. The semiconductor region also generates a signal charge in proportion to the intensity of incident light. The device also comprises a shading membrane having an aperture portion formed on the semiconductor region.
The semiconductor region through which excess signal charge flows and that generates a signal charge in proportion to the intensity of incident light can be made to function as an overflow drain region in addition to generating signal charges (separately from the charge-transfer portion) in proportion to the intensity of light incident thereto through an aperture portion.
It is desirable to further include a read portion that reads out signal charges generated by light incident to the semiconductor region through the aperture portion to the exterior of the semiconductor region.
A solid-state imaging device according to a third embodiment of the present invention comprises a charge-transfer portion that generates and stores signal charges in response to incident light. The charge-transfer portion then transfers the signal charges to an output portion that outputs the signal charges, transferred from the charge-transfer portion, as an electrical signal. The device includes a semiconductor region that generates a signal charge in proportion to the intensity of incident light, and a shading membrane having an aperture portion formed on the semiconductor region. This solid-state imaging device further includes a plurality of aperture areas formed by gathering a plurality of apertures from among the aperture portions. The semiconductor region generates a signal charge in proportion to the intensity of light incident to each aperture area. This causes a signal charge to be generated in proportion to the intensity of light incident to the aperture areas in the semiconductor region in real-time during an exposure.
A solid-state imaging device according to a fourth embodiment of the present invention comprises a plurality of charge-transfer portions that generate and store signal charges in response to incident light and then transfer the signal charges. The device includes an output portion that outputs the signal charges, transferred from the charge-transfer portions, as an electrical signal. The device includes a semiconductor region formed adjacent to each charge-transfer portion. Excess signal charge generated by the charge-transfer portions flows through the semiconductor region which generates a signal charge in proportion to the intensity of incident light. A shading membrane is also provided that has an aperture portion formed on the semiconductor region. The solid-state imaging device further includes a plurality of aperture areas formed by gathering a plurality of apertures from among the aperture portions.
Further, it is desirable to include a read portion that reads out signal charges, generated by light incident to the semiconductor region through the aperture portion, to the exterior of the semiconductor region.
A camera according to a fifth embodiment of the present invention comprises a charge-transfer portion that generates and stores signal charges in response to incident light and then transfers the signal charges. The camera also includes an output portion that outputs the signal charges, transferred from the charge-transfer portion, as an electrical signal. The camera also includes a semiconductor region that generates an electrical current proportional to the intensity of incident light. The camera also includes a shading membrane having an aperture portion formed on the semiconductor region, and a read portion that reads out electrical current, generated by means of light incident to the semiconductor region through the aperture portion, to the exterior of the semiconductor region. The camera further comprises a shutter that blocks light incident to the solid-state imaging device, a current-integration circuit that converts current read from the read portion into voltage, a comparator that compares the voltage to a reference voltage, and a controller that determines, from the output of the comparator, a timing for closing the shutter.
A camera according to a sixth embodiment of the present invention comprises a charge-transfer portion that generates and stores signal charges in response to incident light and then transfers the signal charge; an output portion that outputs the signal charge, transferred from the charge-transfer portion, as an electrical signal; a semiconductor region, formed adjacent to the charge-transfer portion, through which flows excess signal charge generated by the charge-transfer portion and that generates an electrical current proportional to the intensity of incident light; a shading membrane, having an aperture portion, formed on the semiconductor region; and a read portion that reads out electrical current, generated by light incident to the semiconductor region through the aperture portion, to the exterior of the semiconductor region. The camera further comprises a shutter that blocks light incident to the solid-state imaging device; a current-integration circuit that converts current read from the read portion into voltage; a comparator that compares the voltage to a reference voltage; and a controller that determines, from the output of the comparator, a timing for closing the shutter.
The camera can further comprise a strobe that illuminates light onto an object to be photographed, wherein the controller further determines a timing for stopping the generation of light by the strobe.